1. Field of the Invention
The disclosed invention relates to a semiconductor device including an oxide semiconductor.
2. Description of the Related Art
Memory devices including semiconductor elements are broadly classified into two categories: a volatile memory device that loses stored data when power supply is stopped, and a non-volatile memory device that holds stored data even when power is not supplied.
A typical example of a volatile memory device is a DRAM (dynamic random access memory). A DRAM stores data in such a manner that a transistor included in a memory element is selected and charge is stored in a capacitor (e.g., see Patent Document 1).
When data is read from a DRAM, charge in a capacitor is lost on the above-described principle; thus, another writing operation is necessary whenever data is read out. Moreover, charge flows into or out of a capacitor even when a transistor included in a memory element is not selected, due to leakage current between a source and a drain in an off state (off-state current) or the like; therefore, the data storing time is short. For that reason, another writing operation (refresh operation) is necessary at predetermined intervals, and it is difficult to reduce power consumption sufficiently. Furthermore, since stored data is lost when power supply stops, an additional memory device using a magnetic material or an optical material is needed in order to store the data for a long time.
Another example of a volatile memory device is an SRAM (static random access memory). An SRAM holds stored data by using a circuit such as a flip-flop and thus does not need refresh operation. This means that an SRAM has an advantage over a DRAM. However, cost per storage capacity is increased because a circuit such as a flip-flop is used. Moreover, as in a DRAM, stored data in an SRAM is lost when power supply stops.
A structure in which an oxide semiconductor containing indium (In), gallium (Ga), and zinc (Zn) is provided for an active layer of a transistor included in a DRAM has been disclosed (e.g., see Patent Document 2).